1. Field of the Invention
The present invention relates to methods and systems for removing moisture from a vehicle fuel cell during periods of non-operation to prevent degradation by freezing.
2. Background Art
Fuel cells are devices that produce electricity by an electrochemical process without the need for fuel combustion. In the typical fuel cell, hydrogen gas and oxygen gas are electrochemically combined to produce electricity. The hydrogen used in this process may be obtained from natural gas or methanol while air provides the oxygen source. The only by-products of this process are water vapor and heat. Accordingly, fuel cell-powered electric vehicles reduce emissions and the demand for conventional fossil fuels by eliminating the internal combustion engine (e.g., in completely electric vehicles) or operating the engine at only its most efficient/preferred operating points (e.g., in hybrid electric vehicles). However, while fuel cell-powered vehicles have reduced harmful vehicular emissions, they present other drawbacks.
The typical fuel cell, by way of example, includes an anode and a cathode which are separated by a polymeric electrolyte or proton exchange membrane (PEM). Each of the two electrodes may be coated with a thin layer of catalysts such as platinum. At the anode, the hydrogen is catalytically broken down into electron and hydrogen ions (protons). The electron provides the electricity as the hydrogen ion moves through the polymeric membrane towards the cathode. At the cathode, the hydrogen ions combine with oxygen from the air and electrons to form water. In a typical automotive application the high power demands necessitate that a number of fuel cells will be combined together to form a fuel cell stack.
Although present fuel cell technology has shown marked improvement in automobile applications, there is a tendency for water to remain in the fuel cell stack and related components of the system when a vehicle is shut down after operation. Such remaining water has been found to degrade stack performance, especially in cold weather environments when the temperature drops below the freezing point of water. The formation of ice particles may for example degrade the polymeric membrane of a PEM fuel cell or degrade other components in the system such as deionized water lines.
Accordingly, there is a need for an improved fuel cell system and methods for removing water from a fuel cell stack and system before such water can freeze and degrade the fuel cell structures and other system components.